Depression is a kind of mood disorder, and is accompanied by “depressed mood” and “loss of interest or pleasure” as primary symptoms. According to a survey among medical institutions in Japan, more than 700,000 depressive patients were considered to exist in 2008. However, a diagnosis of depression depends in a large part on the subjective view of a physician or psychologist about emotional aspects of a patient or on the subjective view and response of the patient himself or herself, and is hardly considered to be made through an objective determination. Attempts have, therefore, been made in recent years to identify a component in a body fluid of a patient as an objective guideline for the diagnosis of depression.
It has been reported to be able to determine, as a predictive marker for depression, the level of triptophan or its degradation product, the expression level of a specific gene, or the like in a body fluid (Patent Documents 1 and 2). In the meantime, the present inventors found that the ethanolamine phosphate level in blood is useful as a biomarker for diagnosing depression (Patent Document 3).